Leonard Bernstein’s form appears to have been timbre to color: the sound of an instrument, rather than the key or note, carrying the hue. He spoke about the experience publicly, including in the era of his Young People’s Concerts, the television lectures where he made a generation hear structure in music, and accounts of his musicianship describe an unusual sensitivity to music as color and shape.
The evidence is thinner than the best modern cases. Bernstein was never tested, the surviving descriptions are scattered, and some of what circulates about him is repetition between lists. We place him in the self-described tier on the strength of his own public statements, with the honest note that his case has less primary documentation than a Nabokov or a Messiaen.
What is not in doubt is the sensibility: whether or not the colors were clinical synesthesia, Bernstein taught music as a thing you could almost see, and taught America to watch for it.